The present invention generally relates to integrated circuits and more particularly to an improved integrated circuit package which is cost effective and which reduces ground bounce in the integrated circuit when operated.
As is well known, an integrated circuit is formed on a small, substantially planar, piece of semiconductor, such as silicon, known as a die. The die, in addition to the circuits formed therein, includes connection pads on the top surface at the periphery thereof. Before an integrated circuit is ready for use, the die is encapsulated in an insulating package which generally includes a lead frame having a plurality of leads extending externally from the package. The leads are internally connected to corresponding connection pads to permit the integrated circuit die to be coupled to the outside world.
At least one connection pad and a corresponding lead are reserved for a ground connection and at least one connection pad and a corresponding lead are reserved for a supply voltage connection. Such a ground and supply voltage connection are required to permit an operating power supply voltage to be applied to the die.
In the past, and with respect to less expensive, high volume, integrated circuits, the dies have been encapsulated in an insulating housing referred to as a plastic quad flat pack through injection molding and wherein the dies are conductively bonded to a metallic surface. The metallic surface is employed to distribute the power supply voltage applied to the power supply lead and die pad. The ground connection for the dies is established only through the ground leads and pads. Such grounding is generally sufficient in such high volume integrated circuits since such high volume integrated circuits are generally low performance integrated circuits wherein a minimal amount of internal device switching occur in the operation of such integrated circuits.
There are, however, a significant number of high performance integrated circuits such as microprocessor integrated circuits wherein a high degree of internal device switching occurs. Such high performance integrated circuits generally require a better or harder ground than the aforementioned integrated circuits which are encapsulated in the aforementioned plastic quad flat pack packages. This results because the device switching in high performance integrated circuits causes voltage transients which in turn cause what is known as ground bounce wherein the ground voltage is not at zero volts but rather at some finite voltage.
Integrated circuits, such as microprocessor integrated circuits, are generally able to operate properly if the ground bounce is maintained below an acceptable level, such as, for example, 1 volt. When the ground bounce is above the acceptable level, threshold levels of the switching devices can be adversely affected causing the high performance integrated circuits to operate improperly.
In order to cope with the ground bounce problem in high performance integrated circuits, such as microprocessor integrated circuits, extreme measures have been previously taken to maintain the ground bounce below the acceptable limit, of, for example, 1 volt. For example, integrated circuit packages have been employed which are formed from ceramic and include multiple laminations of ceramic and conductive ground planes below the integrated circuit die to sink current and disperse the transient voltages and currents. While such packages are capable of maintaining the ground bounce below the acceptable limits, such as 1 volt, the cost of such packaging has been on the order of ten times the cost of the previously mentioned plastic quad flat pack packages. Plastic quad flat pack packages are not a viable alternative because use of such packages for high performance integrated circuits has been shown to result in a ground bounce which is greater than the acceptable limit, of, for example, 1 volt, and more particularly, in the unacceptable range on the order of 2.5 volts.
The packaging costs of ceramic packages of the type described above have represented a significant portion of the overall costs of high performance integrated circuits. Hence, a significant cost reduction in high performance integrated circuits can therefore be realized with an improved package which represents less manufacturing cost while maintaining the ground bounce within acceptable limits such as, for example, 1 volt or less.